Typical solvent dewaxing processes mix waxy oil feed with solvent from a solvent recovery system. The waxy oil feed/solvent mixture is cooled by heat exchange and filtered to recover solid wax particles. A filtrate comprising a mixture of oil and solvent is recovered from the filtration step. At present, dewaxing of waxy feed is performed by mixing the feed with a solvent to completely dissolve the waxy feed at a suitable elevated temperature. The mixture is gradually cooled to an appropriate temperature required for the precipitation of the wax and the wax is separated on a rotary filter drum. The dewaxed oil is obtained by evaporation of the solvent and is useful as a lubricating oil of low pour point.
This type of dewaxing apparatus is expensive and complicated. In many instances the filtration proceeds slowly and represents a bottleneck in the process because of low filtration rates caused by the high viscosity of the oil/solvent/wax slurry feed to the filter. The high viscosity of the feed to the filter is due to a low supply of available solvent to be injected into the feed stream to the filter. In some cases, lack of sufficient solvent can result in poor wax crystallization and ultimately lower lube oil recovery.
The use of solvents to facilitate wax removal from lubricants is energy intensive due to the requirement for separating from the dewaxed oil and recovery of the expensive solvents for recycle in the dewaxing process.
The solvent is conventionally separated from the dewaxed oil by the addition of heat, followed by a combination of multistage flash and distillation operations. The separated solvent vapors must then be cooled and condensed and further cooled to the dewaxing temperature prior to recycle to the process.
Membrane separation of solvent from the filtrate is a promising process, if suitably selective membranes can be found and operated a low temperature to achieve thermodynamic efficiencies. Such membranes are found in U.S. Pat. Nos. 5,264,166 (White et al) and 5,360,530 (Gould et al); and the present invention relates to improved operation of selectively permeable membranes. These membranes are found to have a high permeability for solvent at low temperature, while rejecting oil, and are suitable for use in solvent recovery from the oil/solvent filtrate mixture.
It has been discovered that the membrane separation can be improved by solvent washing of the membrane under pressurized process conditions.